Breast cancer drug with fewer side effects found to cut risk of tumors

LOS ANGELES--A drug already used to treat breast cancer can reduce the risk of tumors in high- and moderate-risk post-menopausal women by 65 percent over a three-year period, researchers reported Saturday.

Two other drugs are already approved for reducing the risk of breast tumors in healthy women: Generic tamoxifen reduces the risk by 50 percent over a five-year period and raloxifene (Evista) reduces the risk by 38 percent over a similar period.

But both drugs are associated with an increased risk of potentially fatal uterine cancer and blood clots. Fewer than 4 percent of the 2 million women who might benefit from the drugs actually use them.

Exemestane, sold under the brand name Aromasin, provides an even greater reduction in breast cancer risk and so far does not appear to have those potentially lethal side effects, a team headed by Dr. Paul E. Goss of the Massachusetts General Hospital Cancer Center reported at a Chicago meeting of the American Society of Clinical Oncology and online in the New England Journal of Medicine.

Some experts predict that the number of women taking prophylactic therapy should increase significantly, particularly since the drug is already on the market for fighting breast cancer and physicians can prescribe it for any purpose they choose.

“Breast cancer is the second most common cause of death from cancer in women and one of the most feared diagnoses for women in the United States,” Dr. Nancy E. Davidson and Dr. Thomas W. Kensler of the University of Pittsburgh Cancer Institute wrote in an editorial accompanying the report. “We have the knowledge and tools to reduce its incidence today. We have run out of excuses. What are we waiting for?”

But Dr. Joanne Mortimer, director of the Women's Cancers Program at the City of Hope Comprehensive Cancer Center in Duarte, noted that even though all three drugs have been shown to reduce the risk of breast cancer, there is as of yet no evidence that they reduce the risk of dying. That is why “most patients opt not to take them,” she said.

Many studies have shown that estrogen produced by the body promotes breast tumors. Tamoxifen and raloxifene are so-called anti-estrogens that bind to receptors on the surface of breast tissue cells, preventing estrogen from binding.

Exemestane, manufactured by Pfizer Inc., is one of a family of drugs called aromatase inhibitors, which block production of estrogen by the body.

The new study, conducted by Canada's NCIC Clinical Trials Group with partial support from Pfizer, involved 4,560 women from Canada, the U.S., France and Spain with a median age of 62.5 years. The women were all post-menopausal and had at least one other risk factor for breast cancer, such as a benign growth of breast tissues. Half were given exemestane for three years and half a placebo.

After three years, researchers observed 11 invasive breast cancers in the women receiving exemestane, compared with 32 in the group receiving the placebo. There were also significantly fewer precancerous lesions in the group receiving the drug.

“In order to know if it will decrease mortality, that will take years and years and years,” said Dr. Cathie Chung, a medical oncologist at St. John's Health Center in Santa Monica on the west side of Los Angeles. But because so many women fear breast cancer, if researchers can produce a significant reduction in incidence, “that by itself is a significant endpoint.”

The primary side effects among the women in the study were hot flashes, fatigue, sweating, insomnia and bone stiffness, which were all slightly more common in the group receiving the drug. More serious side effects, such as bone fractures and non-breast cancers, were equal in both groups.